Oil wells and gas wells on the land and in the shallow sea are drying up in recent years. For that reason, undersea oil wells and undersea gas wells (hereinafter, undersea oil well and undersea gas well are collectively referred to as “undersea oil well”) of a deep sea have been developed.
In the undersea oil well, a flow line and a riser are arranged from a well mouth arranged on the seafloor to an off-shore platform. A production fluid containing crude oil or natural gas is carried from the undersea oil well to the platform through the flow line or the riser. The flow line means a line pipe laid along the geographical features of the surface of the earth or the undersea surface. The riser means a line pipe arranged in such a way as to rise up in the direction of the platform (in other words, in the upward direction) from the undersea surface.
The production fluid flowing in a steel pipe constructing these line pipes (flow line and riser) has a high pressure. Further, in the case where an operation is stopped, a sea water pressure is applied to the flow line and the riser from the outside. Still further, a repeated stress is also applied to the flow line and the riser by the wave and the sea current. For that reason, a high strength steel pipe having a thick wall has been required for the line pipe such as the flow line and the riser.
However, when a steel pipe has its wall thickness and strength increased, the steel pipe has its toughness reduced and hence easily causes brittle fracture. For that reason, the high strength steel pipe having a thick wall is required to have excellent toughness.
In this way, a steel pipe having a thick wall for an undersea pipeline is required to have high strength and high toughness. A welded steel pipe has a welded portion (seam portion) along a lengthwise direction and hence the welded portion is lower in toughness than a base material. For that reason, a seamless steel pipe is more suitable for the undersea pipeline than the welded steel pipe.
JP9-287028A (Patent Literature 1) proposes a method for producing a seamless steel pipe. In Patent Literature 1, a seamless steel pipe produced by hot rolling is cooled to a temperature not more than an Ar3 transformation point at a cooling rate not less than 80° C./min and then is quenched and tempered. It is described that the seamless steel pipe produced by the producing method of Patent Literature 1 has crystal grains fined and hence has high strength and high toughness.